The successful implementation of a lightwave communication system requires the manufacture of high quality lightguide fibers having mechanical properties sufficient to withstand stresses to which they are subjected. Typically, the fiber has an outside diameter of 0.13 mm and is drawn from a glass preform having an outer diameter of 17 mm. Each fiber must be capable of withstanding over its entire length a maximum stress level equal to the maximum which the fiber will encounter during installation and service. The importance of fiber strength becomes apparent when one considers that a single fiber failure will result in the loss of several hundred circuits.
The failure of lightguide fibers in tension is commonly associated with surface flaws which cause stress concentrations and lower the tensile strength from that of the pristine unflawed glass. The size of the flaw determines the level of stress concentration and, hence, the failure stress. Even micron-sized surface flaws cause stress concentrations which significantly reduce the tensile strength of the fibers.
Long lengths of lightguide fibers have considerable potential strength but the strength is realized only if the fiber is protected with a layer of a coating material such as a polymer, for example, soon after it has been drawn from a preform. This coating serves to prevent airborne particles from impinging upon and adhering to the surface of the drawn fiber which would serve to weaken it. Also, the coating shields the fiber from surface abrasion, which could be inflicted by subsequent manufacturing processes and handling during installation, provides protection from corrosive environments and spaces the fibers in cable structures.
In one process, the coating material is applied by advancing the lightguide fiber through a reservoir in an open cup applicator containing a liquid polymer material. Typically, the fiber enters the reservoir through a free surface, and exits through a relatively small die orifice at the bottom of the reservoir. The coating material is treated, the diameter of the coated fiber is measured and the fiber is taken up by a suitable capstan.
Uniform wetting of the fiber during the coating process is largely affected by the behavior of an entrance meniscus which exists where the fiber is advanced through the free surface of the coating material in the reservoir. As is well known, the wetting characteristics of two materials such as a coating and glass, depend on surface tension and chemical bonds which are developed between the two materials.
The wetting characteristics are affected by a pumping of air into the meniscus. During the coating process, both the fiber surface and the polymer surface are moving at a relatively high speed. The moving surfaces shear the surrounding air, causing it to flow into the point of the meniscus The drawn fiber pulls a considerable amount of air into the coating material as it enters the free surface of the reservoir. Thus in the coating applicator, the entrance meniscus is drawn down with the moving fiber, instead of rising along its surface as it does under static conditions.
It has been found that as the draw rate exceeds about 0.2 meter per second, which is less than the commonly used rate of approximately one meter per second, this pumping action causes the meniscus to extend downwardly and develop essentially into a long, thin column of air which surrounds the fiber and is confined by surface tension in the coating material. Tests have shown that the viscosity of air is sufficiently high to sustain a column of air of considerable depth.
Air entrainment in the form of bubbles on the moving fiber occurs as relatively thin packets of air break off from the column and are carried along with the fiber on its surface. They remain on the fiber, resembling a skin, until they reach a region of pressure gradient in the vicinity of the die opening where they are compressed. This causes the air packets to bulge and form bubbles which may be removed by the surrounding flow lines leading away from the fiber. Should an air packet be compressed farther downstream where all the flow lines extend out of the die with the fiber, the bubble can exit along with the fiber. As the quantity of these bubbles increases, more tend to pass through the die and remain in the coating on the fiber.
As the draw speed is increased, the meniscus becomes unstable, oscillating between a fully developed state with circulation and a relatively small one with little or no circulation. At higher speeds, the column can extend completely through the polymer coating material. In such case, the fiber no longer contacts the polymer, the meniscus collapses and the fiber exits the die with no coating material or with an intermittent, beaded coating.
Bubbles in the fiber coating or beaded coatings may cause several problems. Larger bubbles may extend through the coating thereby exposing the fiber to the environment and to mechanical effects such as abrasion whereas smaller size bubbles cause losses in transmission. In the case of a beaded structure, portions of the fiber may be inadequately coated, while the beads themselves can increase microbending losses when the fiber is packaged into a ribbon assembly. If the fiber is not centered within the coating, portions of its periphery may be inadequately protected from the environment. Instabilities associated with air entrainment can produce fiber misalignment within the coating as well as coating diameter variations. Also, as in the case of smaller bubbles, poor centering can cause transmission losses.
As the coating progresses, bubbles accumulate in the reservoir. It has been found that these bubbles move rapidly with the steamlines in the fluid and coalesce into larger ones. The large, coalesced bubbles interact mechanically with the fiber causing instabilities in fiber alignment with the die.
The prior art has addressed these problems. For example, in one method, the fiber is advanced through an opening in a baffle plate positioned in a reservoir to alleviate or substantially eliminate entrainment of air and consequent bubble formation in the fiber coating. Bubbles are stripped from a region about the fiber due to a hydrodynamic pressure increase in the fluid pressure as the fiber passes through the constriction in the fluid path caused by the baffle plate. Pressure changes are made by changing the geometry of the arrangement such as, for example, the size of the baffle plate opening.
Another coating applicator is shown in U.S. Pat. No. 4,246,299. A fiber is passed through an applicator having a die body that defines a small vertically oriented, longitudinal tapered passage having a reservoir disposed thereabout. A series of radial ports provide fluid communication between the reservoir and the passage. Turbulence within the coating material, which causes entrapment of air bubbles, is reduced by maintaining the level of the coating material in the passage.
In still another apparatus, coating material is directed under pressure radially inward toward a cylindrical passage through which the fiber is advanced. The pressure is maintained sufficently high along the length of the passage to substantially prevent air from entering the passage as the fiber is pulled therethrough. The diameter of the passage is sufficiently large to prevent contact of the fiber with its sides.
In yet another apparatus, there is provided a continuum of liquid coating material, which extends from a free surface in a reservoir through first and second dies at opposite ends of a chamber. The elongated material, which may be drawn lightguide fiber, is advanced through the coating material at a velocity which causes air to become entrained in the coating material and in a direction from the reservoir to the second die. The advance of the elongated material through the coating material causes a pressure gradient to be established between portions of the first die and between the chamber and the second die. The pressure in the coating material decreases from an exit orifice of the first die to the free surface and increases from within the chamber to a location prior to an exit orifice of the second die. Coating material is flowed into the chamber at a pressure which sufficiently enhances the pressure gradient between the portions of the first die to cause air bubbles on the elongated material to be stripped. The introduction of the coating material into the chamber also establishes a volumetric flow of the coating material from the chamber into the reservoir. This is effective to cause the bubbles to be removed and causes the coating material in the chamber and on the coated elongated material to be substantially bubble-free.
As for the familiar extrusion coating of plastic on copper to produce an insulated conductor, the conductor is drawn through a close-fitting core tube, through a die cavity and through the die land wherein a polymer coating is applied at high pressures. However, when coating lightguide fibers, care must be taken to avoid contact of the drawn fiber with the coating apparatus.
Notwithstanding the existence of proffered solutions to the problem of bubbles in lightguide fiber coatings, further improvements in the methods and apparatus which reliably substantially reduce, if not eliminate such bubbles are desirable. Each layer of coating material should be one which is continuous, well-centered about the lightguide fiber and uniformly thick. However, due to the fact that the bubbles which are produced enlarge as they travel toward the reservoir free surface, there is still a tendency for these bubbles to impact the fiber thereby causing misalignment of the fiber in the die.